1. Field of the Invention
The present invention relates to a printed circuit board (PCB) for high frequency on which high frequency circuit elements of a GHz band are mounted and more particularly to a PCB capable of transmitting a signal with a low power consumption.
This application claims priority of Japanese Patent Application No. 2004-058947, filed on Mar. 3, 2004 and Japanese Patent Application No. 2005-007887, filed on Jan. 14, 2005, the entireties of which are incorporated by reference herein.
2. Description of the Related Art
In recent years, according to a request for an information process at high speed and an information communication with high density at high speed, the frequency of a signal for operating a high frequency semiconductor element has been remarkably increased. For instance, an LSI chip currently used for a CPU of a computer operates under the clock frequency of several GHz. Further, in a satellite broadcasting a future expansion of which is greatly anticipated or a mobile communication such as a mobile phone, a mobile terminal or the like, a high frequency signal of a GHz band is used.
In a high frequency circuit, a loss is generated in a conductor and a PCB. Especially, the loss in the PCB has been serious. These losses appear as problems such as a heat generation, noise, high consumed electric power when transmitting a signal. That is, the material of the PCB used for elements in a high frequency band is desirably the material which has a property of low loss for the high frequency signal.
When the PCB tracks with characteristic impedance Zo (Ω) is driven under voltage V (V), a transmission loss P (W) is expressed by a following formula (1). This formula shows that the increase of Zo is effective for decrease of P under the condition of constant V.P=V2/Zo  (1)
The characteristic impedance Zo is proportional to a square root of a ratio of the relative magnetic permeability μr of the material of the PCB to the relative dielectric constant ∈r as shown by a following formula (2).Zo=(L/C)1/2∝(μr/∈r)1/2  (2)(Here, L represents an inductance per unit length of PCB track and C represents an electric capacity per unit length).
As the PCB with the low loss showing the high characteristic impedance Zo, the PCB using a material of low dielectric constant has been hitherto proposed (for instance, see Japanese Patent Application Laid-Open No. hei 6-53357). In this PCB, the characteristic impedance Zo can be increased by decreasing the relative dielectric constant ∈r of the PCB, so that the loss can be decreased.
As the material of the low dielectric constant, a fluororesin such as polytetrafluoro ethylene (the relative dielectric constant ∈r is about 2.1). When the porous density of the above-described fluororesin is increased, a lower dielectric constant can be achieved. For instance, in a porous polytetrafluoro ethylene resin having holes as many as 80%, the relative dielectric constant ∈r is about 1.1.
However, the PCB formed with the porous fluororesin is extremely low in its mechanical strength and has low stability for thermal. Accordingly, the above-described PCB has been hardly available for practical use.
Further, since there does not exist the material whose relative dielectric constant ∈r is smaller than 1, there has been a limitation in a method for reducing the loss of the conventional PCB.